nixos/msmtp: add msmtp module

This adds a module for msmtp, a simple SMTP client which provides a
sendmail-compatible interface.

GitHub: see also nixpkgs issue #105710

nixos/modules/module-list.nix

@@ -140,6 +140,7 @@
   ./programs/light.nix
   ./programs/mosh.nix
   ./programs/mininet.nix
+  ./programs/msmtp.nix
   ./programs/mtr.nix
   ./programs/nano.nix
   ./programs/neovim.nix

nixos/modules/programs/msmtp.nix

@@ -0,0 +1,104 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.programs.msmtp;
+
+in {
+  meta.maintainers = with maintainers; [ pacien ];
+
+  options = {
+    programs.msmtp = {
+      enable = mkEnableOption "msmtp - an SMTP client";
+
+      setSendmail = mkOption {
+        type = types.bool;
+        default = true;
+        description = ''
+          Whether to set the system sendmail to msmtp's.
+        '';
+      };
+
+      defaults = mkOption {
+        type = types.attrs;
+        default = {};
+        example = {
+          aliases = "/etc/aliases";
+          port = 587;
+          tls = true;
+        };
+        description = ''
+          Default values applied to all accounts.
+          See msmtp(1) for the available options.
+        '';
+      };
+
+      accounts = mkOption {
+        type = with types; attrsOf attrs;
+        default = {};
+        example = {
+          "default" = {
+            host = "smtp.example";
+            auth = true;
+            user = "someone";
+            passwordeval = "cat /secrets/password.txt";
+          };
+        };
+        description = ''
+          Named accounts and their respective configurations.
+          The special name "default" allows a default account to be defined.
+          See msmtp(1) for the available options.
+
+          Use `programs.msmtp.extraConfig` instead of this attribute set-based
+          option if ordered account inheritance is needed.
+
+          It is advised to use the `passwordeval` setting to read the password
+          from a secret file to avoid having it written in the world-readable
+          nix store. The password file must end with a newline (`\n`).
+        '';
+      };
+
+      extraConfig = mkOption {
+        type = types.lines;
+        default = "";
+        description = ''
+          Extra lines to add to the msmtp configuration verbatim.
+          See msmtp(1) for the syntax and available options.
+        '';
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    environment.systemPackages = [ pkgs.msmtp ];
+
+    services.mail.sendmailSetuidWrapper = mkIf cfg.setSendmail {
+      program = "sendmail";
+      source = "${pkgs.msmtp}/bin/sendmail";
+      setuid = false;
+      setgid = false;
+    };
+
+    environment.etc."msmtprc".text = let
+      mkValueString = v:
+        if v == true then "on"
+        else if v == false then "off"
+        else generators.mkValueStringDefault {} v;
+      mkKeyValueString = k: v: "${k} ${mkValueString v}";
+      mkInnerSectionString =
+        attrs: concatStringsSep "\n" (mapAttrsToList mkKeyValueString attrs);
+      mkAccountString = name: attrs: ''
+        account ${name}
+        ${mkInnerSectionString attrs}
+      '';
+    in ''
+      defaults
+      ${mkInnerSectionString cfg.defaults}
+
+      ${concatStringsSep "\n" (mapAttrsToList mkAccountString cfg.accounts)}
+
+      ${cfg.extraConfig}
+    '';
+  };
+}